To increase the power of an internal combustion engine (combustion engine), nowadays it is known to use exhaust-gas turbochargers having a compressor, which supplies air for the combustion process to the combustion chamber of the internal combustion engine, and an exhaust-gas turbine in the exhaust tract of the internal combustion engine. The supercharging of the internal combustion engine increases the amount of air and fuel in the combustion chambers (cylinders), and this can result in a significant increase in power of the internal combustion engine. The exhaust-gas turbocharger used for this purpose is normally composed of a rotor, including (e.g., comprising) a compressor wheel and a turbine wheel and the shaft bearing arrangement, the flow-guiding housing parts (compressor housing, turbine housing), and the bearing housing.
During full-load operation of the internal combustion engine, in the exhaust-gas turbocharger, very high circumferential speeds can be attained at the tip diameters of the turbine and compressor wheel. The maximum admissible rotor rotational speed of an exhaust-gas turbocharger is a function of the wheel size, the geometry and the strength values of the materials used. The rotating components can be subjected to very high centrifugal loads and therefore high material stresses. Defects in the material structure can under some circumstances lead to rupture of the compressor or turbine wheel, with in some cases severe consequences for the housing parts which surround the rotating components. The containment concept of an exhaust-gas turbocharger can basically be interpreted as meaning that all fragments are retained within the external housing and do not pose a risk to the surroundings of the supercharger.
In the event a compressor fails as a result of a compressor wheel breaking apart, the braking torque on the turbocharger rotor is eliminated, whereby the now freely driving turbine is accelerated to excessive rotational speeds, and fails when the natural rupture rotational speed is reached. With regard to the natural rupture of a radial turbine, a distinction is made between two types of rupture.
In the case of hub rupture, the entire hub body including the turbine blades disintegrates into multiple fragments, wherein the full rotational energy of the turbine is released instantaneously. The fragments which accelerate outward with high momentum cause considerable damage to the surrounding housing parts and, in the worst case, can even penetrate through said housing parts and thereby pose a risk to the surroundings of the turbocharger.
On the other hand, the turbine can be designed such that it fails through blade rupture. Here, the turbine blades fail in the root region to the hub body, while the wheel hub of the turbine wheel remains intact, continues to rotate and is braked only by friction against the surrounding housing part. Since, in the case of blade rupture, initially only the turbine blades are detached from the wheel hub, initially only the kinetic energy component of the blades is released to the surroundings. During the course of the run-down, the rest of the rotational energy, that is to say the component in the hub, is released to the housing by the stated friction.
In the case of blade rupture of a radial turbine at the natural rupture rotational speed thereof, there can be the problem that, after the blades are thrown off, residual energy remains in the non-rupturing hub body. Rupture tests have shown that, in the case of blade rupture, the shaft can fails and break apart, between the two radial bearings in the region of the axial bearing. In these situations, the turbine-side part of the shaft together with the hub body of the turbine wheel is no longer adequately secured in an axial direction by the axial bearing arranged at the compressor side of the rupture point, and can emerge from the turbocharger housing in the axial direction and pass into the gas outlet line. Here, the friction-welded connection between the shaft and the turbine hub body remains intact. By contrast, the compressor-side part of the shaft remains blocked in the compressor-side bearing point and does not follow the hub body of the turbine wheel and the shaft stub fastened thereto.
A freely rotating component in the gas outlet lines of the turbine is undesirable because this is uncontrollable and can cause damage to the exhaust lines.
EP 1353 041 A1 discloses an exhaust-gas turbocharger in which, on the shaft which is connected to the turbine wheel, there is arranged a means for axially securing the shaft and the turbine wheel connected thereto. In the event of rupture of the compressor wheel, the securing means prevents an axial movement of the shaft, and of the turbine wheel connected thereto, in the direction of the turbine. The securing means is for example, a circlip which is arranged in a groove in the shaft and which, in the installed state, together with housing parts, forms an axial stop for the shaft. The mounting and dismounting of a shaft secured in this manner in the event of maintenance work is cumbersome.